Various types of digital image capture (or image reading) devices are available in the prior art. More particularly, various types of image capture devices are available in the prior art for obtaining a digital image of an original image. For example, image capture devices such as scanners (both handheld and flatbed) for personal computers, digital cameras, facsimile machines, and digital copying machines are available in the prior art. Some of these traditional digital imaging devices perform calibration before each capture of a digital image, upon power up, periodically, or according to some other appropriate regime to ensure that the digital imaging devices are properly calibrated, thereby enabling the digital imaging devices to capture high quality digital images.
Typically, a traditional digital imaging device, such as a flatbed scanner, includes a calibration area therein, which is scanned by the digital imaging device in order to properly calibrate such device. For example, a flatbed scanner typically includes a calibration area, which may comprise an image implemented within a portion of the flatbed scanner, an ink or silk screen, or a sticker having an image thereon, or may simply be a portion of the plastic that forms the scanner's housing, as examples. Such a calibration area is located in a position such that the flatbed scanner's scanner head is capable of scanning the calibration area to calibrate itself. For example, the calibration area may be located at the top or the bottom of the scan bed, depending on how the scan head is positioned when it is idle. To perform calibration, the flatbed scanner scans the calibration area and then based on analysis of the captured image data for the known calibration area, the scanner adjusts/corrects for defects (or quality errors), such as variations of illumination, sensor sensitivity, pixel errors, etcetera. Thus, the flatbed scanner can calibrate itself by scanning a known calibration area that is included within the scanner, and adjusting its imaging (e.g., illumination, etcetera) such that a high quality digital image of the known calibration area is obtained.
A flatbed scanner may also perform a “dark calibration,” wherein the scanner's lamp is turned off such that the calibration area is not illuminated. A scan is performed without the calibration area illuminated, and the scanner ensures that it captures a completely black (or dark) digital image. The sensor (i.e., the image pick-up device) of the scanner may not output a zero signal (indicating no light reflection) when scanning a completely dark area, but may instead output a very small signal (e.g., voltage level). Some sensors are implemented to output large voltage levels to indicate darkness and small voltage levels to indicate light. In either case, the signal actually output by a sensor may not correspond to the expected signal for indicating a very dark or very light area of a scanned original. Accordingly, by performing a calibration scan and analyzing the output of the sensor in comparison with an expected output, the scanner can adjust/calibrate itself to properly interpret reflected light intensities from an original. Offset circuits and/or software may be utilized to adjust the scanner in response to such calibration. Thus, a flatbed scanner may measure the voltage output for each pixel of the known calibration area when the area is illuminated and when very dark to ensure that it is properly calibrated. More specifically, the scanner may store information obtained from such a calibration scan either in the scanner itself or in a computer to which the scanner is coupled, and the appropriate correcting factors are then applied for a subsequent scan of an original to ensure that the digital image captured is an accurate likeness of the original (e.g., appropriate color, no streaks, appropriate darkness, etc.). That is, appropriate adjustments are made in the scanner's hardware and/or software to ensure that the scanner is properly calibrated for scanning an original. For example, the light source of the scanner may not illuminate the edges of an original document with as great an intensity as the center of the original document. If the scanner were not properly calibrated, the captured digital image of the original may show the edges of the original as being dark, when they may in fact be white. As a result, it may be difficult to perform further image processing, such as optical character recognition (OCR) operations, of such an inaccurate digital image. However, by properly calibrating the scanner, it can properly compensate for such a “roll-off” of light on the edges of the document, for example, and capture a digital image having an accurate likeness of the original.
More recently, look-down digital imaging devices have been developed. In general, look-down digital imaging devices include a digital imaging device that looks down at an original positioned face up below the digital imaging device, and the digital imaging device captures a digital image of such original. As an example of such a look-down digital imaging device of the prior art, digital document cameras have recently become commercially available. For instance, a prior art digital document camera is commercially available from Canon, such as Canon's digital document camera model DZ-3600U. Further examples of prior art look-down digital imaging devices include those disclosed in U.S. Pat. No. 5,227,896 issued to Takashi Ozawa entitled “IMAGE READER FOR PRODUCING AND SYNTHESIZING SEGMENTED IMAGE DATA” and U.S. Pat. No. 5,515,181 issued to Tetsuo Iyoda entitled “IMAGE READING APPARATUS PROVIDING HIGH QUALITY IMAGES THROUGH SYNTHESIS OF SEGMENTED IMAGE DATA.”
However, look-down digital imaging devices of the prior art generally do not include a mechanism for calibrating such devices. As described above, flatbed scanners typically include a calibration area that is utilized in a manner that accurately imitates the reflected path of light when scanning an original. However, such a calibration path is generally not available in look-down digital imaging devices. One possible technique for calibrating a look-down digital imaging device of the prior art is for a user to place a calibration image on the target scan area below the look-down digital imaging device and then scan such calibration image in order to calibrate the look-down digital imaging device. However, this technique requires action by the user to perform calibration of the look-down digital imaging device. That is, the user is required to place a calibration image on the target scan area and initiate the calibration process. Accordingly, such a calibration technique would be inefficient, inconvenient and undesirable to a user, especially if calibration of the look-down digital imaging device is desired before every scan of an original. Furthermore, because such calibration technique relies on a scan of an image external to the look-down digital imaging device, such calibration technique is likely not as reliably accurate as is desired. That is, this calibration technique is not performed within such a controlled environment as is available within the digital imaging device itself. Thus, external factors such as the calibration image being improperly positioned on the target area or the calibration image being dirty or otherwise damaged, as examples, may prevent the look-down digital imaging device from being properly calibrated.